The present invention relates to lane following vehicle control technique for controlling a vehicle along a lane of a road.
A Published Japanese Patent Application Kokai Publication No. 7(1995)-104850 discloses a driver assist system for controlling a steering torque for lane following.
In this driver assist system, the steering torque during the lane following control is limited to a relatively low upper limit to allow a driver to readily perform intervening steering operation.
An actuator torque TAC produced by a steering actuator including an electric motor is expressed as:
TACMAX=TSTMAXxe2x88x92FOAxe2x88x92TLOxe2x88x92TSAxe2x80x83xe2x80x83(1)
where TACMAX is a maximum torque produced by the actuator, TLO is a torque loss in the actuator, FAO is an overall friction force in the steering system, TSA is a self aligning torque, and TSTMAX is a maximum steering torque during lane following control. While maximum steering torque TSAMAX is a fixed value which can be determined experimentally, and steering system""s overall friction FAO is also a fixed value calculated by accumulating variations of parts, self aligning torque TSA is a variable varying in accordance with a vehicle running condition.
In the straight ahead running state of the vehicle, self aligning torque TSA is equal to zero, so that it is possible to limit the actuator maximum torque TACMAX accurately to a level enabling driver""s steering intervention. During cornering operation or lane changing operation in which nonzero self aligning torque TSA is produced, however, it is difficult or practically impossible to limit the actuator maximum torque TACMAX accurately to the level for allowing driver""s steering intervention because actuator maximum torque TACMAX is varied by self aligning torque TSA. 
Moreover, in a steering system provided with a vehicle speed responsive type power steering system separately from an actuator for lane following control, a steering assist torque produced by the power steering system decreases with increase in the vehicle speed, and hence the self aligning torque TSA is influenced by the characteristic of the steering assist torque in the power steering system. Therefore, it is difficult or practically impossible to limit the actuator maximum torque TACMAX accurately to the level for allowing driver""s steering intervention because actuator maximum torque TACMAX is varied by self aligning torque TSA. 
It is therefore an object of the present invention to provide lane following vehicle control apparatus and/or method allowing a driver to intervene adequately irrespective of changes in self aligning torque and improving lane following control performance.
According to the present invention, a lane following vehicle control apparatus for a vehicle, comprises: (1) a lane sensing section to collect information on a lane of a road ahead of the vehicle; (2) a steering angle sensing section to sense an actual steering angle of the vehicle; (3) a steering torque producing section to produce an actual steering torque in accordance with a target steering torque to follow the lane; (4) a steering torque controlling section to set the target steering torque required for the controlled vehicle to follow the lane, in accordance with the information on the lane and the actual steering angle; (5) a control steering direction discriminating section to determine whether a control steering direction of the steering torque controlling section is a steer direction away from a straight ahead neutral position or a return direction toward the straight ahead neutral position; and (6) a target steering torque limiting section to limit the target steering torque by setting a target torque limit in dependence on whether the control steering direction is the steer direction or the return direction.
According to the present invention, a lane following vehicle control process comprises: (1) collecting input information on a vehicle running condition of a vehicle running along a lane, and an actual steering angle of the vehicle; (2) producing a control signal to produce an actual steering torque in accordance with a target steering torque to follow the lane; (3) setting the target steering torque to achieve a target steering angle determined in accordance with the vehicle running condition and the actual steering angle to follow the lane; (4) determining whether a control steering direction of the target steering angle is a steer direction away from a straight ahead neutral position or a return direction toward the straight ahead neutral position; and (5) limiting the target steering torque by setting a target torque limit in dependence on whether the control steering direction is the steer direction or the return direction.
According to another aspect of the present invention, a lane following vehicle control apparatus comprises: (1) first means for collecting input information on a vehicle running condition of a vehicle running along a lane; (2) second means for sensing an actual steering angle of the vehicle; (3) third means for producing an actual steering torque in accordance with a target steering torque to follow the lane; (4) fourth means for determining a target steering angle in accordance with the vehicle running condition and the actual steering angle to follow the lane; (5) fifth means for setting the target steering torque to achieve the target steering angle to follow the lane; (6) sixth means for determining a control steering direction by monitoring changes in the target steering angle is; and (7) seventh means for limiting the target steering torque by setting a target torque limit in dependence on whether the control steering direction is a steer direction away from a straight ahead neutral position or a return direction toward the straight ahead neutral position.
The other objects and features of this invention will become understood from the following description with reference to the accompanying drawings.